Understanding Timing

[Ultrasummer]

Howdy folks. I'm trying to set the timing on my truck. 1968 Chevy c10 with a 307 with manual transmission. I read in one place where the factory specs suggest BTDC at zero. And I read in another place where 12 is the sweet spot. This is at warmed up idle. So when I get ready ready to get down to work, I take a look at my timing light target and see this:

[Ultrasummer]

So my question is, how many is each line?

[Boog]

2*

[RustyPile]

The "long" lines are 10 degrees apart. Two degrees between the "short" lines...

[Ultrasummer]

Thanks. And I would guess, the A is advance? Does A in advance = B in BTDC? And R is retard?

[68 P.O.S.]

That is correct.

[Ultrasummer]

awesome. One step closer. Thanks

[68 P.O.S.]

Also, the sweet spot is your total timing, not the initial. Total timing is more important. Shoot for 36 degrees btdc. You'll need a dial back timing light or an timing tape to set it. However, it depends on how many degrees of advance your distributor provides (not including the vacuum advance). Which distributor are you running?

[AussieinNC]

Dont trust the timing notch on the balancer unless its a reasonably new balancer...

I always check TDC is correct first using a positive stop in the #1 spark plug hole....

BY HAND !!!! turn the engine clockwise until the positive stop touches the top of piston #1,,,,mark the balancer at the zero mark on the tab....

BY HAND !!!....turn the engine counter clockwise until the positive stop touches the top of piston #1,,,,mark the balancer at the zero mark on the tab....

Remove the positive stop and replace the #1 plug and lead.

Measure the distance between these two lines and it theoretically should fall on the notch...dont be surprised if it doesnt....mark the mid point between the two original lines and that is true TDC....

Use that new mark as your timing "notch"...

Suggest 12 degrees A at idle (650 rpm) and 30 degrees A all in by 3000 rpm...if it pings, back it off a little...

Have fun...

[68 P.O.S.]

This too ^^^

[truckster]

BTDC is before top dead center. Any timing BTDC is considered advanced.

[Ultrasummer]

Great info. Makes sense. Yeah I had wondered how in the world I was going to be able to get to 36 degrees on that mark on the engine. A friend loaned me his timing light with the dial on it and he told me just to keep it at zero. Now I know what that control knob is for (even though I don't know how to use it) Youtube here I come. Thanks again for all the great advice

[68 P.O.S.]

How did this end up working out for you?

[Ultrasummer]

Well I think we got the timing down. I've come to the conclusion that my 50 yr old Rochester 2G needs rebuilt. Idles fine, sounds good, but when you step into the accelerator it cuts out and dies. Maybe I'm going down the wrong road but I figure 50 yrs, that carb had a pretty good run. I went to Summit Racing and bought a Holley 2 barrel carb with 350 cfm. That's going on this weekend. Then I'll go back and take a look at that timing once again. At least it makes sense now.

[68 P.O.S.]

There you go. Good work. Yes, that carb has seen better days I'm sure haha. After you get the new carb in, you shouldn't have to redo the timing. You should look at re-curving the distributor tho.

[Ultrasummer]

thanks

[geezer#99]

Holley carb eh!
Didja get an adaptor too?

[Ultrasummer]

I did

[Ultrasummer]

Just to clarify :
Find true TDC, and set timing for 12 degrees at idle. Then do total timing at, let's say 36 degrees for example. So when I'm setting the total timing and I have my timing light set at 36, am I looking for the marks to line up at zero or at where 12 was during the initial timing at idle? Also, it seems odd to me that you'd make one adjustment (initial timing) and then go to a second adjustment (total timing) negating what you just did with the initial timing. Does Total Timing trump initial timing? If so, why do initial timing at all? Sorry if this is basic info for everyone else, but I'm just trying to understand completely.

[68 P.O.S.]

Read this article for your timing questions. It was written by a GM Engineer and explains everything pretty well. http://www.camaros.org/pdf/timing101.pdf

Looks like you basically have it right. You just have to remember everything works together. Find TDC the correct way (using a piston stop! Not a stick, finger, paper towel, or whatever) so you know you're dead on correct. If it lines up with the factory timing mark on the balancer, perfect. If not, make a new mark at your new found TDC location on the balancer. Unplug the vacuum advance hose and plug both connections to ensure no vacuum leaks that could affect your numbers. Then set your initial timing anywhere between 8-14, it doesn't really matter where, you just want it to run so you can set the total timing.

Next, the distributor needs to be curved correctly. However, we need to know which distributor you have, points or HEI, and what brand? Ideally you want the distributor to have 20 to 22 degrees of mechanical advance from the weights and centerplate. Remove distributor cap and remove the advance springs carefully, ensuring the weights are lubed and fully extend in their travel by hand. Put the cap back on, and start her up. With the springs out, the weights will fully advance with little rpm. You should only have to rev the engine no more than 1500 rpm. Set the total timing to 36 degrees btdc and you’re done. You don’t worry about initial timing again since your total timing is set optimally. Shut her down, pop open the distributor cap again, and replace the advance springs with new advance springs that allow your fully advanced timing of 36 degrees btdc to come in between 2500-3000 rpm (Crane has a great kit for this). Next is setting up your vacuum advance. Do you know how many degrees of advance it provides?

[Ultrasummer]

No idea. But that all gives me a better idea about everything. I've read the GM engineer's write up before. Thanks for your input. I appreciate it.

[68 P.O.S.]

You're welcome, glad I could help!

[68 P.O.S.]

Here's another great article he's written as well: http://www.corvetteactioncenter.com/...e_hinckley.pdf

[toolboxchev]

Try this, It will explain timing in detail.

https://www.crankshaftcoalition.com/...ategory:Engine

With an older vehicle you will run into other issues. Old stuff likes to break, and you will see things that are not correct. I always fix stuff before catastrophe happens.